Light adapted photoconductive elements



United States Patent Int. Cl. G03g 13/22 US. Cl. 96-1 Claims ABSTRACT OFTHE DISCLOSURE Photoconductive elements which have been exposed to lightprior to charging produce images which have increased resolutions.

This invention relates to electrophotography, and in particular tophotoconductive elements and the process for making and using them.

The process of xerography, as disclosed by Carlson in US. 2,297,691,employs an electrophotographic element comprising a support materialbearing a coating of a normally insulating material whose electricalresistance varies with the amount of incident actinic radiation itreceives during an imagewise exposure. The element, commonly termed aphotoconductive element, is first given a uniform surface charge,generally in the dark after a suitable period of dark adaptation. It isthen exposed to a pattern of actinic radiation which has the effect ofdifferentially reducing the potential of the surface charge inaccordance with the relative energy contained in various parts of theradiation pattern. The differential surface charge or electrostaticlatent image remaining on the electrophotographic element is then madevisible by contacting the surface with a suitable electroscopic markingmaterial. Such marking material or toner, whether contained in aninsulating liquid or on a dry carrier, can be deposited on the exposedsurface in accordance with either the charge pattern or the dischargepattern as desired. The deposited marking material can then be eitherpermanently fixed to the surface of the sensitive element by known meanssuch as heat, pressure, solvent vapor, or the like, or transferred to asecond element to which it can similarly be fixed. Likewise, theelectrostatic latent image can be transferred to a second element anddeveloped there.

Various photoconductive insulating materials have been employed in themanufacture of electrophotographic elements. For example, vapors ofselenium and vapors of selenium alloys deposited on a suitable supportand particles of photoconductive zinc oxide held in a resinous,film-forming binder have found wide applications in present-day documentcopying applications.

Since the introduction of electrophotography, a great many organiccompounds have also been screened for their photoconductive properties.As a result, a very large number of organic compounds are known topossess some degree of photoconductivity. Many organic compounds haverevealed a useful level of photoconduction and have been incorporatedinto photoconductive compositions. Optically clear organicphotoconductor-containing elements having desirable electrophotographicproperties can be especially useful in electrophotography. Suchelectrophotographic elements can be exposed through a transparent baseif desired, thereby providing unusual flexibility in equipment design.Such compositions, when coated as a film or layer on a suitable supportalso yield an element which is reusable; that is, it can be used to formsubsequent images after residual toner from prior images has beenremoved by transfer and/or cleaning.

3,533,783 Patented Oct. 13, 1970 Particularly useful are photoconductorswhich display little or no persistence of photoconductivity. This classof photoconductors includes those which are rechargeable and do notretain any traces of the original image after having been exposed to apattern of actinic radiation and recharged. Therefore, photoconductorswhich have this property are reusable. Included in the class are organicincluding organo-metallic, as well as some inorganic, photoconductingcompounds.

In many xerographic applications it is desirable for the reproduction tohave a relatively high resolution as measured in terms of lines permillimeter. A typical application where high resolution images arenecessary is in microfilm reproductions. Ideally, a microfilmduplicating system should provide exact duplicates of existing microfilmframes with no loss in resolution from the original.

It is therefore, an object of this invention to provide a new processfor preparing photoconductive elements which upon exposure produceimages having high resolution.

It is another object of this invention to provide a new process forusing the photoconductive elements to produce images having highresolution.

It is a further object of this invention to provide novelphotoconductive elements which exhibit high resolving power.

It is also an object of this invention to provide novel photoconductiveelements which after exposure to an image retain their resolving powerfor substantially long periods of time prior to development.

Yet another object of this invention is to provide a new process forpreparing photoconductive elements which upon exposure to an image canbe stored in the dark for substantially long periods of time prior todevelopment without a significant loss of resolution in the finaldeveloped image.

These and other objects of this inventon are accomplished by subjectinga photoconductive element con taining a photoconductor which displayssubstantially no persistence of photoconductivity to an amount of lightfor a period of time sufficient to increase the resolution of the finalimage prior to charging and its use in recording images. By adapting theelement to light prior to charging and imagewise exposing, the resultantrecorded image has a higher resolution than that obtainable in theabsence of a light adaptation step. Also, after the element is exposedin an imagewise manner, the light adapted layer retains its increaseresolving power for a substantial period of time prior to developing asopposed to a loss in the resolving power in an element which has notbeen light adapted. Therefore, before developing, the element can remainin the dark for at least about one hour or more without significantlyaffecting the resolution of the image.

The process of this invention includes the following steps:

1) Preparing the element including coating at least one photoconductivelayer on a conducting support and drying,

(2) Light adapting the element by subjecting the element to an overalllight exposure for a period of time,

(3) Charging the element,

(4) Exposing the element in an imagewise manner and (5) Developing theresultant latent image.

The light adaptation step (Step 2 above) takes place after the elementis prepared. The element can be light adapted merely by subjecting it toa source of light such as ordinary room light or sunlight for a periodof time sufiicient to increase the resolution of the final image. Higherlight intensities require shorter exposure times than lower intensities.Generally, the minimum amount of light necessary to obtain the increaseresolutions of this invention is about foot-candle-seconds While themaximum amount of light is dependent upon the stability of theparticular photoconductive composition employed. However, it is notgenerally practical to employ more than about 10 foot-candle-secondssince the increase in resolution thus obtained is slight compared to theincrease in the amount of light. Also, the stabilities of the variousphotoconductors are not significantly effected by this amount of light.The preferred exposure to which the photoconductive elements aresubjected is from about 6.l 10 foot-candle-seconds to about 6.1 10foot-candle-seconds.

After the element has been light adapted, it is preferable to charge andexpose it as soon as possible. If the element remains in the dark for along period of time after the light adaptation step, its resolving poweris somewhat diminished. For example, an element which has been lightadapted at an exposure of 6.1 X 10 foot-candleseconds begins to lose itsresolving power if placed in the dark for more than 24 hours.

The electrophotographic elements to be used in this invention can beprepared in the usual manner, i.e., by blending a dispersion or solutionof a photoconductive compound together with a binder, when necessary ordesirable, and coating or forming a self-supporting layer with thephotoconductor-containing material. Mixtures of the photoconductorsdescribed herein can be employed. Likewise, other photoconductors knownin the art can be combined with the present photoconductors. Inaddition, supplemental materials useful for changing the spectralsensitivity or electrophotosensitivity of the element can be added tothe composition of the element when it is desirable to produce thecharacteristic effect of such materials.

The increased image resolutions of this invention can be obtained byUsing an organic including organo-metallie, or inorganic,photoconducting material which has little or substantially nopersistence of photoconductivity. A typical inorganic photoconductorwhich has the required lack of persistence is selenium whilerepresentative organo-metallic compounds are the organic derivatives ofGroup IVa and Va metals such as those having at least one amino-arylgroup attached to the metal atom. Exemplary organo-metallic compoundsare the triphenylp-dialkylaminophenyl derivatives of silicon, germanium,tin and lead and the tri-p-dialkylaminophenyl derivatives of arsenic,antimony, phosphorus and bismuth.

An especially useful class of organic photoconductors is referred toherein as organic amine photoconductors. Such organic photoconductorshave as a common structural feature at least one amino group. Usefulorganic photoconductors which can be spectrally sensitized in accordancewith this invention include, therefore, arylamine compounds comprising(1) diarylamines such as diphenylamine, dinaphthylamine,N,N-diphenylbenzidine, N phenyl-l-naphthylamine;N-phenyl-Z-naphthylamine; N,N diphenyl-p-phenylenediamine;2-carboxy-5-chloro- 4 methoxydiphenylamine; panilinophenol; N,N-di-2-naphthyl-p-phenylenediamine; 4,4'-benzylidene-bis-(N,N-diethyl-m-toluidine), those described in Fox US. Pat. 3,240,597 issuedMar. 15, 1966, and the like, and (2) triarylamines including (a)nonpolymeric triarylamines, such as triphenylamine,N,N,N,N-tetraphenyl-m-phenylenediamine; 4-acetyltriphenylamine,4-hexanoyltriphenylamine; 4-lauroyltriphenylamine;4-hexyltriphenylamine, 4- dodecyltriphenylamine,4,4-bis(diphenylamino)benzil, 4, 4-bis(diphenylamino)benzophenone, andthe like, and (b) polymeric triarylamines such as poly[N,4"-(N,N,N-triphenylbenzidine)]; polyadiplytriphenylamine,polysebacyltriphenylamine; polydecamethylenetriphenylamine; poly N(4-vinylphenyl)diphenylamine, poly-N-(vinylphenyl)-a,a'-dinaphthylamineand the like. Other useful 4 amine-type photoconductors are disclosed inUS. Pat. 3,180,730 issued Apr. 27, 1965.

Useful photoconductive substances capable of being spectrally sensitizedin accordance with this invention are disclosed in Fox US. Pat.3,265,496 issued Aug. 9, 1966, and include those represented by thefollowing general formula:

wherein A represents a mononuclear or polynuclear divalent aromaticradical, either fused or linear, (e.g., phenyl, naphthyl, biphenyl,binaphthyl, etc.), or a substituted divalent aromatic radical of thesetypes wherein said substituent can comprise a member such as an acylgroup having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl,butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms(e.g., methyl, ethyl, propyl, butyl,

' etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g.,methoxy, ethoxy, propoxy, pentoxy, etc.), or a nitro group; A representsa mononuclear or polynuclear monovalent aromatic radical, either fusedor linear (e.g., phenyl, naphthyl, biphenyl, etc.); or a substitutedmonovalent aromatic radical wherein said substituent can comprise amember, such as an acyl group having from 1 to about 6 carbon atoms(e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), analkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy,propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogenatom, a halogen atom or an aromatic amino group, such as A'NH; brepresents an integer from 1 to about 12, and G represents a hydrogenatom, a mononuclear or polynuclear aromatic radical, either fused orlinear (e.g., phenyl, naphthyl, biphenyl, etc.), a substituted aromaticradical wherein said substituent comprises an alkyl group, an alkoxygroup, an acyl group, or a nitro group, or a poly(4'-vinylpl1enyl) groupwhich is bonded to the nitrogen atom by a carbon atom of the phenylgroup.

Polyarylalkane photoconductors are particularly useful in producing thepresent invention. Such photoconductors are described in US. Pat.3,274,000; French Pat. 1,383,461 and in copending application of Sensand Goldman titled Photoconductive Elements Containing OrganicPhotoconductors filed Apr. 3, 1967 or Ser. No. 627,857. Thesephotoconductors include leuco bases of diaryl or triaryl methane dyesalts, 1,1,l-triarylalkanes wherein the alkane moiety has at least twocarbon atoms and tetraarylmethanes, there being substituted an aminegroup on at least one of the aryl groups attached to the alkane andmethane moieties of the latter two classes of photoconductors which arenon-leuco base materials.

Preferred polyarylalkane photoconductors can be represented by theformula:

wherein each of D, E and G is an aryl group and J is a hydrogen atom, analkyl group, or an aryl group, at least one of D, E and G containing anamino substituent. The aryl groups attached to the central carbon atomare preferably phenyl groups, although naphthyl groups can also be used.Such aryl groups can contain such substituents as alkyl and alkoxytypically having 1 to 8 carbon atoms, hydroxy, halogen, etc. in theortho, meta or para positions, ortho-substituted phenyl being preferred.The aryl groups can also be joined together or cyclized to form afluorene moiety, for example. The amino substituent can be representedby the formula wherein each L can be an alkyl group typically having 1to 8 carbon atoms, a hydrogen atom, an aryl group, or together thenecessary atoms to form a heterocyclic amino group typically having 5 to6 atoms in the ring such as morpholino, pyridyl, pyrryl, etc. At leastone of D, E, and G is preferably p-dialkylarninophenyl group. When I isan alkyl group, such an alkyl group more generally has 1 to 7 carbonatoms.

Representative useful polyarylalkane photoconductors include thecompounds listed below:

Table A Compound No.2

Compound Name 1 4,4'-benzylidine-bis(N,N-diethyl-m-toluidine). (2) 4,4"diamino-4-dimethylamino 2',2" dimethyl-triphenylmethane. (3) 4,4"bis(diethylamino) 2,6-dichloro-2',2"-

dimethyltriphenylmethane. (4) 4',4" bis(diethylamino) 2',2"dimethyldiphenyl-naphthylmethane. (5 2',2" dimethyl 4,4',4-tris(dimethylamino) triphenylrnethane. (6) 4,4"- bis (diethylamino)-4-dimethylamino-2,

2"-dimethyltriphenylmethane. (7) 4,4 bis(diethylamino) 2chloro-2,2"-dimethyl-4-dimethylaminotriphenylmethane. (8)4',4-bis(diethylamino)-4-dimethylamino-2,2,

2"-trimethyltriphenylmethane. (9) 4',4" bis (dimethylamino-2-chloro-2',2"-dimethyltriphenylmethane. l) 4,4" bis(dimethylamino)2',2"-dimethyl4- methoxy-triphenylmethane. 11)-.Bis(4-diethylamino)-1,1,1-triphenylethane. 12) Bis(4-diethylamino)tetraphenylmethane. (13) 4',4"-bis(benZylethylamino);2,2"-dimethyltriphenylmethane. (l4) 4,4" bis(diethylamino) 2,2diethoxytriphenylmethane. 1 4,4-bis (dirnethylamino) -1,1,1-triphenylethane. (16) 1-(4N,N-dimethylaminophenyl)-1,1-diphenylethane. 17)4-dimethylaminotetraphenylmethane. (18) 4-diethylaminotetraphenylmethane.

Another class of photoconductors useful in this invention are the4-diarylamino-substituted chalcones. Typical compounds of this type arelow molecular weight nonpolymeric ketones having the general formula:

R1 wherein R and R are each phenyl radicals including substituted phenylradicals and particularly when R is a phenyl radical having the formula:

where R and R are each aryl radicals, aliphatic residues of 1 to 12carbon atoms such as alkyl radicals preferably having 1 to 4 carbonatoms or hydrogen. Particularly advantageous results are obtained when Ris a phenyl radical including substituted phenyl radicals and where R;is diphenylamino, dimethylamino or hydrogen.

Sensitizing compounds useful with the photoconductive elements of thepresent invention can include a wide variety of substances such aspyrylium, thiapyrylium, and selenapyrylium salts of US. Pat. 3,250,615,issued May 10, 1966; fluorenes, such as 7,12-dioxo 13 dibenzo-(a,h)fluorene, 5,10 dioxo 4 a,11 diazabenzo (b)fluorene, 3,13 dioxo 7oxadibenzo (b,g)fluorene, trinitrofluorenone, tetranitrofluorenone andthe like; aromatic nitro compounds of US. Pat. 2,610,120; anthrones ofUS. Pat. 2,670,285; quinones of U.S. Pat. 2,670,286; benzophenones ofUS. Pat. 2,670,287; thiazoles of US. Pat. 2,732,301; mineral acids;carboxylic acids, such as maleic acid, dichloroacetic acid, andsalicylic acid; sulfonic and phosphoric acids; and various dyes such astriphenylrnethane, diarylmethane, thiazine, azine, oxazine, xanthene,phthalein, acridine, azo, anthraquinone dyes and many other suitablesensitizing dyes. The preferredsensitizers for use with the compounds ofthis invention are pyrylium and thiapyrylium salts, fluorenes,carboxylic acids, and triphenylrnethane dyes.

Where a sensitizing compound is to be used within a photoconductivelayer as disclosed herein it is conventional practice to mix a suitableamount of the sensitizing compounds with the coating composition sothat, after thorough mixing, the sensitizing compound is uniformlydistributed throughout the desired layer of the coated element. Inpreparing the photoconducting layers, no sensitizing compound is neededfor the layer to exhibit photoconductivity. The lower limit ofsensitizer required in a particular photoconductive layer is, therefore,zero. However, since relatively minor amounts of sensitizing compoundgive substantial improvement in the electrophotographic speed of suchlayers, the use of some sensitizer is preferred. The amount ofsensitizer that can be added to a photoconductor-incorporating layer togive effective increases in speed can vary widely. The optimumconcentration in any 'given case will vary with the specificphotoconductor and sensitizing compound used. In general, substantialspeed gains can be obtained where an appropriate sensitizer is added ina concentration range from about 0.0001 to about 30 percent by weightbased on the weight of the film-forming coating composition. Normally, asensitizer is added to the coating composition in an amount by weightfrom about 0.005 to about 5.0 percent by weight of the total coatingcomposition.

Preferred binders for use in preparing the present photoconductivelayers are film-forming polymeric binders having fairly high dielectricstrength which are good electrically insulating film-forming vehicles.Materials of this type comprise styrene-butadiene copolymers; siliconeresins; styrene-alkyd resins; silicone-alkyl resins; soya-alkyl resins;poly(vinyl chloride)); poly(vinylidene chloride); vinylidenechloride-acrylonitrile copolymers; poly(vinyl acetate); vinyl acetatevinyl chloride copolymers; poly(vinyl acetals), such as poly(vinylbutyral); polyacrylic and methacrylic ester, such aspoly(methylmethacrylate), po1y(n-butylmethacrylate), poly(isobutylmethacrylate), etc.; polystyrene; nitrated polystyrene;polymethylstyrene; isobutylene polymers; polyesters, such aspoly(ethylenealkaryloxyalkylene terephthalate); phenolforrnaldehyderesins; ketone resins; polyamide; polycarbonates; polythiocarbonates;poly(ethyleneglycolcobishydroxyethoxyphenyl propane terephthalate); etc.Methods of making resins of this type have been described in the priorart, for example, styrene-alkyd resins can be prepared according to themethod described in US. Pat. 2,361,019 and 2,258,423. Suitable resins ofthe type contemplated for use in the photoconductive layers of theinvention are sold under such tradenames as Vietel PE-101, Cymac,Piccopale 10 0, Saran F220 and Lexan 10.5. Other types of binders whichcan be used in the photoconductive layers of the invention include suchmaterials as paraffin, mineral waxes, etc.

Solvents of choice for preparing coating compositions of the presentinvention can include a number of solvents such as benzene, toluene,acetone, Z-b-utanone, chlorinated hydrocarbons, e.g., methylenechloride, ethylene chloride, etc., ethers, e.g., tetrahydrofuran, ormixtures of these solvents, etc.

In preparing the coating composition useful results are obtained Wherethe photoconductor substance is present in an amount equal to at leastabout 1 weight percent of the coating composition. The upper limit inthe amount of photoconductor substance present can be widely varied inaccordance with usual practice. In those cases where a binder isemployed, it is normally required that the photoconductor substance bepresent in an amount from about 1 weight percent of the coatingcomposition to about 99 weight percent of the coating composition. Apreferred weight range for the photoconductor substance in the coatingcomposition is f m about 10 weight percent to about 60 weight percentCoating thicknesses of the photoconductive composition on a support canvary widely. Normally, a coating in the range of about 0.001 inch toabout 0.01 inch before drying is useful for the practice of thisinvention. The preferred range of coating thickness was found to be inthe range from about 0.002 inch to about 0.006 inch before dryingalthough useful results can be obtained outside of this range. Aspreviously mentioned, more than one layer may be coated on the support.Good results are obtainable When a first layer containing aphotoconductor, a binder and a sensilizer is overcoated with a secondlayer of a composition containing a photoconductor and a binder. Thephotoconductor and binder employed in the overcoat can be different thanthose employed in the first layer.

Suitable supporting materials for coating the photoconductive layers ofthe present invention can include any of a wide variety of electricallyconducting supports, for example, paper (at a relative humidity abovepercent); aluminum-paper laminates; metal foils such as aluminum foil,zinc foil, etc.; metal plates, such as aluminum, copper, zinc, brass,and galvanized plates; vapor deposited metal layers such as silver oraluminum and the like. An especially useful conducting support can beprepared by coating a support material such as polyethyleneterephthalate with a layer containing a semiconductor dispersed in aresin. Such conducting layers both with and Without insulating barrierlayers are described in U.S. Pat. 3,245,833. Likewise, a suitableconducting coating can be prepared from the sodium salt of acarboxyester lactone of maleic anhydride and a vinyl acetate polymer.Such kinds of conducting layers and methods for their optimumpreparation and use are disclosed in U.S. 3,007,901 and 3,267,807.

The elements of the present invention can be employed in any of thewell-known electrophotographic processes which require photoconductivelayers. One such process is the aforementioned xerographic process. Aspreviously explained, in a process of this type the electrophotographicelement is given a blanket electrostatic charge by placing the sameunder a corona discharge which serves to give a uniform charge to thesurface of the photoconductive layer of at least 400 volts andpreferably at least 500 volts. This charge is retained by the layerowing to the substantial insulating property of the layer, i.e. the lowconductivity of the layer in the dark. The electrostatic charge formedon the surface of the photoconducting layer is then selectivelydissipated from the surface of the layer by exposure to light through animagebearing transparency by a conventional exposure operation such as,for example, by contact-printing technique, or by lens projection of animage, etc., to form a latent image in the photoconducting layer. Byexposure of the surface in this manner, a charged pattern is created byvirtue of the fact that light causes the charge to be conducted away inproportion to the intensity of the illumination in a particular area.The charge pattern remaining after exposure is then developed, i.e.,rendered visible, by treatment with a medium comprisingelectrostatically attractable particles having optical density. Thedeveloping electrostatically attractable particles can be in the form ofa dust, e.g., powder, pigment in a resinous carrier, i.e., toner, or aliquid developer may be used in which the developing particles arecarried in an electrically insulating liquid carrier. Methods ofdevelopment of this type are widely known and have been described in thepatent literature in such patents, for example, as U.S. Pat. 2,297,691and in Australian Pat. 212,315. In processes of electrophotographicreproduction such as in xerography, by selecting a developing particlewhich has as one of its components, a low-melting resin, it is possibleto treat the developed photoconductive material with heat and cause thepowder to adhere permanently to the surface of the photoconductivelayer. In other cases, a transfer of the image formed on thephotoconductive layer can be made to a second support, which would thenbecome the final print. Techniques of the type indicated are Well knownin the art and have been described in a number of U.S. and foreignpatents, such as U.S. Pats. 2,297,691 and 2,551,582, and in RCA Review,vol. 15 (1954), pages 469-484.

The present invention is not limited to any particular mode of use ofthe new electrophotographic materials, and the exposure technique, thecharging method, the transfer (if any), the developing method, and thefixing method as well as the materials used in these methods can beselected and adapted to the requirements of any particular technique.

Electrophotographic materials according to the present invention can beapplied to reproduction techniques wherein different kinds ofradiations, i.e., electromagnetic radiations as well as nuclearradiations, can be used. For this reason, it is pointed out herein thatalthough materials according to the invention are mainly intended foruse in connection with methods comprising an exposure, the termelectrophotography wherever appearing in the description and the claims,is to be interpreted broadly and understood to comprise both xerographiyand xeroradiography.

The invention is further illustrated by the following examples whichinclude preferred embodiments thereof.

EXAMPLE 1 A pyrylium sensitized triphenylamine organic photoconductor ina polyester resin binder, a copolymer made from isoand terephthalicacids, ethylene glycol and 2,2- bis(4-fl-hydroxy-ethoxyphenyl)propane,(overcoated with unsensitized triphenylamine in a polystyrene binder,coated on a Rem-jet backed cellulose triacetate support subbed with asodium OERL (carboxy ester resin lactone) coating, (see Minsk U.S. Pats.2,861,056 and 3,007,901) is used as the recording film for Xerographicmicrofilming in a camera operation. The photoconductor is stored in roomlight for 10 hours. This corresponds to an equivalent exposure of 6.l 10foot-candle-seconds since room light is equal to about 17 foot-candles.The recording film is placed in the dark immediately prior to use and ischarged to a uniform negative surface potential of at least about 500volts. The charged layer is placed in a microfilming camera and exposedto an original for 15 seconds. The exposed photoconductor, representinga 12X reduction of the original, is developed by cascading a combinationof l20-mesh iron filings and a toner powder with a maximum diameter sizeof 5 microns, over its surface. The dry toner is prepared by mixingcarbon pigment and piccolastic Record resin (homologs of polystyrene),heat fusing the mixture, and after ball milling for about 48 hours,sieving through a 200-mesh screen, and air elutriating the powder toproduce a dry toner having the maximum particle size of 5 microns. Theresultant image after development is a positive-appearing micro image ofa positive-appearing original and has a resolving power of lines per mm.The image is fixed to the photoconductor by heat.

EXAMPLE 2 A second xerographic image made by the above method istransferred to a moistened, partially hardened gelatin layer on atransparent film base instead of being fixed by heat. This image alsohas a resolving power of 100 lines per mm.

EXAMPLE 3 A photoconductive element is prepared in the same manner asthat in Example 1 except that the element is stored for 24 hours in thedark instead of being exposed to light prior to charging. The element isthen charged, exposed and developed in the same manner as set forth inExample 1. The resolution of the resultant image is 30 lines per mm.

. EXAMPLE 4 The procedure of Example 1 is followed except that after theimage exposure step the element is allowed to remain in the dark for onehour prior to development. The resolving power of the resultant image is100 lines per mm. Thus, when this result is compared with the result ofExample 1, it can be seen that the light adapted layer not only producesa higher initial resolving power but is also capable of retaining saidresolving power for at least one hour.

EXAMPLE 5 The procedure of Example 3 is followed except that after theimage exposure step the element is allowed to remain in the dark for onehour before development. The resolving power of the resultant image islines per mm. When this result is compared with the result of Example 3,it is seen that the dark adapted sample not only shows poor initialresolving power, but also allows the latent image to degenerate duringthe one hour dark decay period.

EXAMPLE 6 This example is carried out in a manner similar to that setforth in Example 1 except that the photoconductor,4,4'-diethylamino-2,2'-dimetbyltriphenylmethane, is exposed to roomlight for 24 hours prior to charging. After the element is exposed in animagewise manner, it is placed in the dark for one hour prior todeveloping. The latent image is then developed with a liquid developeraccording to conventional techniques. The final image has a resolvingpower of 251 lines per millimeter. Without the light adaptation step,the image has a resolving power of 158 lines per millimeter.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be etfected within the spirit and scope of theinvention as described hereinbefore and as defined in the appendedclaims.

I claim:

1. A process for increasing the resolution of a recorded xerographicimage comprising:

(a) subjecting a photoconductive element to a light exposure of at leastabout 6.1x 10 foot-candle-seconds, said element comprising aphotoconductive composition containing an organic photoconductor whichhas substantially no persistence of photoconductivity coated on aconducting support,

(b) charging the light adapted element to at least 400 volts,

(c) exposing the charged, light adapted element to actinic radiation inan imagewise manner and (d) developing the resultant latentelectrostatic image with a liquid developer.

2. The process of claim 1 including the step of allowing the element toremain in the dark for at least about one hour after exposing it toactinic radiation and before developing.

3. The process of claim 1 wherein the photoconductive element comprisesa conducting support having coated thereon a layer of a compositioncomprising a binder, a sensitizer and an organic photoconductor which isovercoated by a layer of a composition comprising a binder and anorganic photoconductor.

4. The process of claim 1 wherein the photoconductive element issubjected to a light exposure of about 6.1 X 10 foot-candle-seconds.

5. A process for increasing the resolution of a recorded xerographicimage comprising:

(a) a light adapting a photoconductive element containing triphenylamineand a binder by subjecting it to a light exposure of about 6.1 1Ofoot'candleseconds,

(b) charging the light adapted element to at least 500 volts,

(0) exposing the charged element to actinic radiation in an imagewisemanner and (d) developing the resultant recorded xerographic image witha liquid developer.

6. A process for increasing the resolution of a recorded xerographicimage comprising:

(a) subjecting a photoconductive element to a light exposure of at leastabout 6.1 l0 foot-candle-seconds, said element comprising aphotoconductive composition containing an organic photoconductor whichhas substantially no persistence of photoconductivity coated on aconducting support,

(b) charging the light adapted element to at least 400 volts,

(c) exposing the charged, light adapted element to actinic radiation inan imagewise manner and (d) developing the resultant latentelectrostatic image with a dry developer.

7. The process of claim 6 including the step of allowing the element toremain in the dark for at least about one hour after exposing it toactinic radiation and before developing.

8. The process of claim 6 wherein the photoconductive element comprisesa conducting support having coated thereon a layer of a compositioncomprising a binder, a sensitizer and an organic photoconductor which isovercoated by a layer of a composition comprising a binder and anorganic photoconductor.

9. The process of claim- 6 wherein the photoconductive element issubjected to a light exposure of about 6.1 10 foot-candle-seconds.

10. A process for increasing the resolution of a recorded xerographicimage comprising:

(a) light adapting a photoconductive element containing triphenylamineand a binder by subjecting it to a light exposure of about 6.1 10foot-candleseconds,

(b) charging the light adapted element to at least 500 volts,

(c) exposing the charged element to actinic radiation in an imagewisemanner and (d) developing the resultant recorded xerographic image witha dry developer.

References Cited UNITED STATES PATENTS 2,845,348 7/1958 Kallman 9612,990,280 6/ 1961 Giaimo 961 3,084,061 4/1963 Hall 11717.5 3,249,4305/1966 Metcalfe 96-1 3,385,699 5/1968 Honjio 96l GEORGE F. LESMES,Primary Examiner I. C. COOPER III, Assistant Examiner s. 01. X.R.96-1.5; 117 17.s, 37; 2s2-so1

